The demand for continued improvement in the efficiency of combustion turbine and combined cycle power plants has driven the designers of these systems to specify increasingly higher turbine inlet temperatures. Although nickel and cobalt based superalloy materials are now used for components in the hot gas flow path, such as combustor transition pieces and turbine rotating and stationary blades, even these superalloy materials are not capable of surviving long term operation at temperatures sometimes as high as 1400.degree. C.
It is known in the art to coat a superalloy metal component with an insulating ceramic material to improve its ability to survive high operating temperatures, for example U.S. Pat. No. 4,321,310 (Ulion et al). It is also known to coat the insulating ceramic material with an erosion resistant material to reduce its susceptibility to wear caused by the impact of particles carried within the hot gas flow path; for example, U.S. Pat. Nos. 5,683,825 and 5,562,998 (Bruce, et al. and Strangman, respectively).
Much of the development in this field of technology has been driven by the aircraft engine industry, where turbine engines are required to operate at high temperatures, and are also subjected to frequent temperature transients as the power level of the engine is varied. A combustion turbine engine installed in a land-based power generating plant is also subjected to high operating temperatures and temperature transients, but it may also be required to operate at full power and at its highest temperatures for very long periods of time, such as for days or even weeks at a time. Prior art insulating systems are susceptible to degradation under such conditions at the elevated temperatures demanded in the most modern combustion turbine systems.
U.S. Ser. No. 09/245262, filed on Feb. 2, 1999 (Subramanian, et al.; ESCM 283139-00491), also related to columnar thermal barrier coatings (TBCs), usually of yttria-stabilized zirconia (YSZ), deposited by electron beam physical vapor deposition (EB-PVD) with a sintering resistant layer of aluminum oxide or yttrium aluminum oxide, deposited as a continuous or discontinuous layer between submicron gaps in the TBC columns. This material was thermally stable up to about 1200.degree. C. Other columnar TBC coatings are described in U.S. Ser. No. 09/393,415, filed on Sep. 10, 1999, (Subramanian; ESCM 283139-00224) , where TBC columns had a composition of (A,B).sub.x O.sub.y and were covered by a sheath of a composition of C.sub.Z O.sub.W, where A,B and C were selected from Al, Ca, Mg, Zr, Y, Sc and rare earth equal to La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb. In this application, a reaction between C.sub.z O.sub.w and (A,B).sub.x O.sub.y was key to obtain a multiphase TBC system which was expected to be sinter resistant and strain tolerant up to 1400.degree. and higher. The same materials were used as an (A,B).sub.x O.sub.y planar based TBC coated with a C.sub.z O.sub.w overlay in U.S. Ser. No. 09/393,417, filed on Sep. 10, 1999, (Subramanian; ESCM 283139-00223). In this application also, a reaction between C.sub.z O.sub.w and (A,B).sub.x O.sub.y was key to obtain a multiphase TBC system which was expected to be sinter resistant and strain tolerant up to 1400.degree. and higher. Specific compounds capable for application as TBCs are described in U.S. Ser. No. 09/405,498, filed on Sep. 24, 1999 (Subramanian, et al.; ESCM 283139-00076). There, TBC layers of LaAlO.sub.3, NdAlO.sub.3, La.sub.2 Hf.sub.2 O.sub.7, Dy.sub.3 Al.sub.5 O.sub.12, Ho.sub.3 Al.sub.5 O.sub.12, ErAlO.sub.3, GdAlO.sub.3, Yb.sub.2 Ti.sub.2 O.sub.7, LaYbO.sub.3, Gd.sub.2 Hf.sub.2 O.sub.7, and Y.sub.3 Al5O.sub.12 were generally described. These were compounds capable for TBC application, due to their inherently superior sintering resistance and phase stability.
A solid, vapor deposition material useful for the EB-PVD method to provide heat resistant coatings in aircraft engines and the like, where excellent heat resistance and thermal shock resistance is required, is taught by U.S. Pat. No. 5,789,330 (Kondo, et al). There, the material is sintered zirconia, containing a special stabilizer selected from yttria, magnesium oxide, calcium oxide, scandium oxide, or oxides of rare earth elements equal to La, Ce, Pr, Nd, Pm, Sm, Eu, Hd, Tb, Dy, fermium, Wr, thulium, Yb and ruthenium in the range of 0.1 wt percent to 40 wt percent of the material. The sintered material has 25% to 70% monoclinic phase and up to 3% tetragonal phase, with the rest as cubic phase.
Some high temperature resistant coatings, as taught in U.S. Pat. No. 5,304,519 (Jackson, et al), have utilized thermal spraying of zircon plus zirconia particles (ZrSiO.sub.4 and ZrO.sub.2 respectively) partially stabilized with an oxide selected from CaO, Y.sub.2 O.sub.3, MgO, CeO.sub.2, HfO.sub.2 or rare earth oxide, where rare earth equal La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. These materials are used as refractory, thermal shock resistant coatings for hearth rolls for annealing steel, stainless steel and silicon steel sheet at furnace temperatures between 820.degree. C. and 1100.degree. C.
Data regarding sintering rates of single oxides A.sub.x O.sub.y are available but only a few publications discuss sintering rates of multicomponent oxides. One such publication is by Shinozaki, et al. 1981, (9), pp. 1454-1461, where the sintering tendencies of a solid solution of mixed Sm.sub.2 O.sub.3 --ZrO.sub.2 were discussed in The Chemical Society of Japan, "Sintering Sm.sub.2 O.sub.3 --ZrO.sub.2 Solid Solution." There, tablets of the mixed component oxides at various mole % were sintered at from 1200.degree. C. to 1600.degree. C. and isothermal linear shrinkage was measured. The least amount of sintering, 3% to 10% at 1400.degree. C., was found at ranges of 5 mole % to 50 mole % Sm.sub.2 O.sub.3.
In "La.sub.2 Zr.sub.2 O.sub.7 --a new candidate for thermal barrier coatings", R. Va.beta.en, X. Cao, F. Tietz, G. Kerkhoff, D. Stover, United Thermal Spray Conference, 17.-19.3.99, Dusseldorf, Hrsg. E. Lugscheider, P. A. Kammer, Verlag Fur Schwei.beta.en und Verwandte Verfahren, Dusseldorf, 1999, p. 830-034, plasma sprayed TBC coatings of one specific compound, La.sub.2 Zr.sub.2 O7, were discussed. Although this material is of the pyrochlore structure, as shown in their FIG. 2, our own results in the Example, below, show this specific compound is not good as a TBC. However, introduction of cation excess/defects or oxygen defects change the sintering properties and this is not suggested in the paper.
What is needed is a TBC coating for a device, where the coating will remain thermally stable, protective, strain compliant, and resistant to substantial sintering of gaps in its grain structure, for use in long-term, high temperature turbine applications at temperatures up to 1400.degree. C. Preferably the TBC will be a new material which itself meets the above criteria without the need for extra processing steps or additional coating.
It is a main object of this invention to provide a device which is capable of operating at temperatures up to about 1400.degree. C. for extended periods of time with reduced component degradation. It is a further object of this invention to provide a method of producing such a device that utilizes commercially available materials processing steps.